1. Field of the Invention
The present invention relates to a method for fabricating a filament type high-temperature superconducting wire, and more particularly to a method for fabricating a filament type high-temperature superconducting wire in which a thin film type high-temperature superconducting wire is fabricated into a filament shape suitable for use with alternating current.
2. Description of the Related Art
Superconductive phenomenon refers to which resistance of a substance disappears when the temperature of the substance is lowered. That is, when a substance exhibiting superconductive phenomenon is used electricity can flow without generating heat, thus there is no energy loss. Such a substance is called a superconductor. Superconductive phenomenon is exhibited in certain substances and not influenced by temperature, magnetic field, or current. A superconductor can flow a current without resistance only under the superconductive transition temperature (Tc) and critical magnetic field (Bc). Under these circumstances, the critical current density (Ic), which is the highest current density capable of flowing current without resistance, exists.
In incorporating a superconductor, it is advantageous to fabricate the superconductor into a linear or tape shape and in use by the same applicable equipments manufactured by fabricating transmission lines superconductive magnets and the like can be used. In the case of a thin film type high-temperature superconducting wire, it is made into a tape shape having small thickness. Conventionally, the structure of a thin film type high-temperature superconducting wire is fabricated by coating a single layer or multi-layers of a ceramic layer on a metal substrate having a tape shape, coating a superconductive layer followed by a metal layer on the ceramic layer, as shown in FIG. 1A. Metal substrate used herein is a bi-axially textured metal substrate or a metal substrate having no specifically orientated texture. In the latter case, a bi-axially textured ceramic layer is coated on the metal substrate using ion beam assisted deposition (IBAD). The thin film type high-temperature superconducting wire is fabricated by preparing a metal substrate coated with one or two or more ceramic layers epitaxially on a metal substrate having a bi-axially textured surface, and coating a superconductive layer epitaxially on the ceramic layer. On the surface of the fabricated thin film type high-temperature superconducting wire, a metal layer having good conductivity is coated to thermally and mechanically protect the high-temperature superconducting thin film. Thus, the final thin film type high-temperature superconducting wire is completely formed.
FIG. 1B illustrates the structure of a thin film type high-temperature superconducting wire fabricated by coating a single layer or multi-layers of a ceramic layer on a metal substrate having no specifically oriented texture, coating and IBAD layer having bi-axially textured property followed by a ceramic layer, and then coating a superconducting layer and a metal layer sequentially thereon.
The bi-axially textured metal substrate is prepared via rolling a metal or a metal alloy having a face-centered cubic structure followed by heat treatment, and has a tape shape with small thickness and wide width. Similarly, the metal substrate, which has no specifically oriented texture, while having a bi-axially textured ceramic thin film coated by ion beam assisted deposition or an IBAD layer having bi-axially textured property also has a tape shape with small thickness and wide width. Therefore, the final thin film type high-temperature superconducting wire also has a tape shape with small thickness and wide width.
A superconducting wire can be incorporated into electric power lines, motors, transformers, fault current limiters, MRI magnets, NMR magnets and the like, in these uses, a direct current or alternating current power is used. When using a superconducting wire with alternating current, the direction of the current changes continuously such as to generate AC loss, thereby power loss occurs. Therefore, it is very important to reduce the AC loss of the superconducting wire. As a method for reducing the AC loss of the superconducting wire into a filament shape and twisting the filament may reduce the AC loss dramatically. Also, a multiple core wire is used as the superconducting wire for the stability of the superconductor (Lawrence Dresner, “Stability of Superconductor,” Plenum Press, New York, N.Y. (1995)). Superconductive current flows through the superconducting filament in the multiple core superconducting wire.
However, as directed above, in the conventional method for fabricating a think film type high-temperature superconducting wire, there are problems in that it is difficult to use the wire in a twisted form and the AC loss is big because the final shape of the wire has a tape shape with small thickness and wide width; the disadvantage in the formability is generated when incorporating the wire into a solenoid coil, a pancake coil, or a cable; and the cross-sectional area of the superconducting wire occupying the total cross-sectional area of the wire is small.